1. Technical Field
The present invention relates to liquid crystal display devices, and particularly to a type including a pair of opposing substrates between which a liquid crystal layer is disposed, and a pair of electrodes separated by an insulating layer that are disposed on one of the substrates and drive the liquid crystal layer.
2. Related Art
Among liquid crystal display devices, a twisted nematic (TN) type is widely used. This type is limited in viewing angle in principle. Transverse electric field liquid crystal display devices are known as a type to eliminate the limitation. In the transverse electric field type, pixel electrodes and a common electrode are formed on the same substrate, and a voltage is applied between the pixel electrodes and the common electrode to generate an electric field substantially parallel to the surface of the substrate, thereby driving liquid crystal molecules in a plane substantially parallel to the surface of the substrate.
Transverse electric field liquid crystal display devices include an in-plane switching (IPS) type and a fringe field switching (FFS) type. In the IPS type, a comb-like pixel electrode and another comb-like common electrode are arranged in combination. In the FFS type, an upper electrode and a lower electrode are disposed with an insulating layer in between. One of the electrodes is used as a common electrode while the other is used as a pixel electrode, and apertures through which an electric field is transmitted, such as slits, are formed in the upper electrode.
Japanese Unexamined Patent Application Publication No. 2001-183685 discloses a structure of the insulating layer between the upper electrode and the lower electrode. In this structure, the pixel electrode and the common electrode are formed of ITO with an insulating layer in between. The insulating layer is defined by a TFT surface protection insulating layer or a TFT gate insulating layer.
A liquid crystal display device includes a storage capacitor to reduce the changes in pixel potential for driving the liquid crystal. In the FFS type, the capacitor defined by the overlap of the upper electrode and the lower electrode can be used as the storage capacitor, with the insulating layer disposed between the upper electrode and the lower electrode. For a smaller storage capacitor having a higher capacitance, the insulating layer between the upper electrode and the lower electrode requires increased properties.
For a FFS type liquid crystal device, transistors are formed on a glass substrate, and subsequently a planarizing layer is formed. Then, a lower electrode, an insulating layer, and an upper electrode are formed in that order. Since the insulating layer is formed among such many steps for forming layers, the adhesions between the insulating layer and the adjoining layers may be varied to cause the insulating layer to separate. Consequently, problems occur that the liquid crystal display device cannot display high-quality images. For example, the insulation between the upper electrode and the lower electrode may be reduced; the electric field for driving the liquid crystal layer may be disturbed; or the storage capacitor is seriously degraded in characteristics.